This invention relates to a communication method and, more particularly, to a method for communicating through a leased line in a virtual private network and a leased line.
VPN (Virtual Private Network) and IN (Intelligent Networks) are kinds of private communication service through a public network. Specified subscribers communicate with one another through the virtual private network or the intelligent networks. Exchangers are incorporated in those networks, and steer pieces of data between the specified subscribers. When a terminal calls another terminal, the exchangers establish a communication path in the virtual private network or the intelligent networks, and pieces of data are transferred through the communication path between the terminals. In the following description, both VPN and IN are simply referred to as xe2x80x9cvirtual private networkxe2x80x9d.
On the other hand, a leased line is exclusively used by a specific user, and an exchanger of the public network does not participate the communication between terminals of the specific user. The leased line achieves a high quality communication rather than the circuit switching service, and the communication through the leased line is more economical rather than the communication through the public network.
If the specified subscribers of the virtual private network have the leased line, the public network and the leased lines are theoretically available for the communication. In the following description, if a leased line is provided in parallel to a part of a virtual private network, the communication system is referred to as xe2x80x9ccomposite communication networkxe2x80x9d. The present inventor searched documents already published for the composite communication network. The present inventor found Japanese Patent Publication of Unexamined Application Nos. 4-291595, 6-205137, 7-183945, 7-221838 and 8-195814. The present inventor checked the Japanese Patent Publication of Unexamined Applications to see how the leased line is used in the composite communication network. However, the leased line is not effectively used.
FIG. 1 illustrates a prior art composite communication network. The virtual private network is owned by a user group 1, and a code (ggggg) is assigned to the user group 1. Subscribers 1a/1b/1c belong to the user group 1. The user group has a head office H1 in a city X and a branch office B1 in another city Y, and private branch exchangers 2a/2b are placed in the head office H1 and the branch office B1, respectively. The subscribers 1b and 1c are in the head office H1 and the branch office B1, and are connected to the private branch exchangers 2a/2b, respectively. The subscriber 1a is in yet another city Z. The communication between the head office H1 and the branch office B1 is so much that a leased line 3 is connected between the private branch exchangers 2a and 2b. 
The virtual private network is established in a public network 4, and the public network 4 has exchangers 4a/4b/4c. The cities X/ Y/ Z are in the service areas of the exchangers 4a/4b/4c, respectively. The exchanger 4c serves as a service switching point, and is under the control of a service control point SCP.
When the subscriber 1a wants to communicate with the subscriber 1c through the virtual private network, the subscriber 1a requests the exchanger 4c to establish a communication path to the subscriber 1c in the virtual private network. The exchanger or the service switching point requests the service control point SCP to route the communication path from the subscriber 1a to the subscriber 1c. The service control point SCP takes the service area of the exchanger 4b into account, and routes the communication path 5 as xe2x80x9cExchanger 4cxe2x80x9dxe2x80x94xe2x80x9cExchanger 4axe2x80x9dxe2x80x94xe2x80x9cExchanger 4bxe2x80x9dxe2x80x94xe2x80x9cPBX 2bxe2x80x9dxe2x80x94xe2x80x9cSubscriber 1cxe2x80x9d. Even though the leased line 3 is connected between the private branch exchangers 2a and 2b, the leased line 3 is not used for the communication between the subscribers 1a and 1c. 
As described hereinbefore, the leased line provides a high quality communication, and is more economical rather than the public network. Therefore, the communication through the leased line 3 is desirable. However, the leased line 3 is not always used for the communication.
It is therefore an important object of the present invention to provide a method for a composite communication network which is a higher in quality and more economical than a virtual private network.
In accordance with one aspect of the present invention, there is provided a method for routing a communication path between a first subscriber and a second subscriber through a public network having service switching points and a service control point and a private network containing entry points and a leased line selectively connected between the entry points, and the method comprises the steps of transferring a first identification number and a second identification number respectively assigned to the first subscriber and second subscriber through at least one of the service switching points to the service control point, determining one of the entry points closest to the first subscriber and a routing number used in the private network, notifying the routing number to the one of the entry points through the public network for routing a communication sub-path passing through the leased line in the private network and establishing the communication path having the communication sub-path between the first subscriber and the second subscriber.